CAM DESIGN GUIDELINES

18.0 INTRODUCTION

This brief chapter attempts to put in summary form some of the lessons “learned the hard way” over 40 years of cam and machine design experience. Some myths and old shibboleths are also debunked. It is the author’s impression that there is still a lot of misinformation and misunderstanding in the machine design world about the design of cam follower systems. It is hoped that these general guidelines will be of some help in improving the situation.

18.1 PRACTICAL DESIGN CONSIDERATIONS

The cam designer is often faced with many confusing decisions, especially at an early stage of the design process. Many early decisions, often made somewhat arbitrarily and without much thought, can have significant and costly consequences later in the design. The following is a discussion of some of the trade-offs involved with such decisions in the hope that it will provide the cam designer with some guidance in making these decisions.

Translating or Oscillating Follower?

There are many cases, especially early in a design, when either translating or rotating motion could be accommodated as output from the cam, though in other situations, the follower motion and geometry is dictated to the designer. If some design freedom is allowed, and straight-line motion is specified, the designer should consider the possibility of using an approximate straight line motion, which is often adequate and can be obtained from a large-radius rocker follower. The rocker or oscillating follower has advantages over the translating follower when a roller is used. A round-cross-section translating follower slide is free to rotate about its axis of translation and needs to have some antirotation guiding provided (such as a keyway or second slide) to prevent z -axis misalignment of the roller follower with the cam. Many commercially available, nonrotating slide assemblies are now available, often fitted with ball bearings, and these provide a good way to deal with this issue. However, an oscillating follower arm will keep the roller follower aligned in the same plane as the cam with no guiding other than its own pivot.

Also, the pivot friction in an oscillating arm follower has a small moment arm compared to the moment of the force from the cam acting on the follower arm at the roller. Conversely, the friction force on a translating follower has a one-to-one geometric relationship with the cam force. This can have a larger parasitic effect on the system.

Another advantage of oscillating arm followers over translating followers is their ability to accommodate somewhat larger pressure angles since there is less tendency to jam the follower than with a sliding joint. This allows a smaller cam diameter, all else equal. However, due to their low friction, ball bearing slides used for translating followers can also allow somewhat larger maximum pressure angles than the traditional 30 ° limitation.

Translating flat-faced followers are often deliberately arranged with their axis slightly out of the plane of the cam in order to create a rotation about their own axis due to the frictional moment resulting from the offset. The flat follower will then precess around its own axis and distribute the wear over its entire face surface. This is common practice with automotive valve cams that use flat-faced followers or “tappets.”